Advanced #49

The electrical conductivity of water reaches a maximum at 230 degrees Celsius

The intrinsic conductivity of pure water has a maximum at high temperature.

Scientific Explanation

Pure water is not a perfect insulator — it always contains small quantities of protons (H-plus) and hydroxide ions (OH-minus) produced by autoprotolysis: two water molecules exchange a proton, forming a hydronium ion and a hydroxide ion. The concentration of these ions and their mobility determine the intrinsic electrical conductivity.

With rising temperature, autoprotolysis increases — more ions form, and conductivity grows. At the same time, ion mobility also rises with temperature. Up to approximately 230 degrees Celsius (under sufficient pressure to keep water liquid), these two effects prevail, and conductivity climbs.

Above 230 degrees Celsius, however, the dielectric constant drops so severely that water can no longer stabilize the ions as effectively. The ion concentration decreases, and despite continued increases in mobility, the conductivity falls. The maximum at 230 degrees Celsius thus marks the point where the decline of the dielectric constant just overtakes the rise in thermal energy.

Electrical Conductivity of Pure Water vs Temperature Line chart showing that the intrinsic electrical conductivity of pure water increases with temperature and reaches a maximum at about 230 degrees C, then decreases at higher temperatures as the dielectric constant drops too much to support ion formation. Conductivity (S/m) 10⁻¹ 10⁻² 10⁻³ 10⁻⁴ 0 100 200 300 400 Temperature (°C) Maximum ~230 °C Intrinsic Conductivity of Pure Water
Intrinsic conductivity of pure water with a maximum at about 230 degrees Celsius.

Everyday Relevance

This maximum is relevant for power plants and geothermal energy. In steam turbines and pressurized water reactors, water circulates at temperatures around 300 degrees Celsius. The conductivity maximum at 230 degrees means that in this range the corrosiveness of the water is particularly high, since the elevated ion concentration promotes electrochemical reactions at metal surfaces. Engineers must carefully control the water chemistry in these systems to minimize corrosion.